My stock Raspbian system has
libswscale2 available in the repositories, but OSMC does not. How do the repositories differ?
$ cat /etc/apt/sources.list deb http://mirror.internode.on.net/pub/raspbian/raspbian jessie main contrib non-free rpi
$ cat /etc/apt/sources.list deb http://mirrordirector.raspbian.org/raspbian jessie main contrib non-free rpi deb http://apt.osmc.tv jessie main
I actually manually added
rpi to the OSMC file, as it was missing earlier, but this did not help.